stracener_emis 7305/5305_spr08_04.22.08 1 system availability modeling & analysis case studies...
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System Availability Modeling & AnalysisCase Studies
Dr. Jerrell T. Stracener, SAE Fellow
Leadership in Engineering
EMIS 7305/5305Systems Reliability, Supportability and Availability Analysis
Systems Engineering ProgramDepartment of Engineering Management, Information and Systems
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Aircraft Availability Analysis
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Aircraft Availability Model Logic
InitiateScenario Fly Mission
Profile
ReadyAircraft
Available?
SubstituteAvailable?
Repair andReturn to Ready
Pool
In-FlightGripe?
Defect?
Repair OnAircraft?
SpareAvailable?
Trouble Shoot @2-Digit WUC Level
Service andTurnaround Aircraft
Perform RepairMTL/EMT/MM
Wait for Spare Repair Item
Remove ItemReplace Unit
and Check Out
Yes
No
Yes
Yes
Yes
Yes
Yes
No
No
No
No
No
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Blue Flame Aircraft Case Study
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Blue Flame Availability Analysis
• Previous availability & support system analysis applications (heritage)
• Review of Blue Flame Requirements and system/subsystem characteristics
• Determination of radar component of Blue Flame availability
• Development of Blue Flame radar availability model• Calculation of Blue Flame radar baseline availability
estimates
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Blue Flame Analysis Working Definitions
• System Reliability Design Characteristics– Mean-Time-Between-Failure (MTBF)-a reliability function which assumes that
operation occurs after early failure (infant mortality) and prior to wear-out, I.e., a constant failure rate exists.
– Mean-Time-Between-Maintenance-Actions (MTBMA)-a reliability function which accounts for all causes of maintenance activity, whether a failure occurred or not.
• System Maintainability Design Characteristics– Mean-Time-To-Repair (MTTR)-a maintenance function, can include corrective
maintenance time (CMT) and preventive maintenance time (PMT)
• Support System Design Characteristics– Mean-Logistics-Down-Time (MLDT)-a maintenance related logistics function
which involves spares provisioning and logistics delay time (LDT) and administrative delay time (ADT)
Operational Availability (Ao)- the degree to which an item
(the radar set) is in an operable and committable state at the start of a mission when the mission is called for at a random time.
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Blue Flame Operational Availability• Inherent Availability (Ai)
Ai =
• Achieved Availability (Aa)
Aa =
• Operational Availability (Ao)
MTBF
MTBF + MTTR(CMT)
MTBF
MTBF + MTTR
MTBF
MTBF + MTTR + MLDTAo =
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Blue Flame Fleet Requirements
• Fleet Requirements – Operational Availability -- 95%
– Sortie Rate --12/PAA/Mo (Peacetime)– Mission Reliability --93% (High Mission)
--96% (Low Mission)
• Fleet Operational Data – 3.5 flying hrs/high mission --50% of missions– 1.5 flying hrs/low mission --50% of missions
– 500 aircraft -- one radar set per aircraft
– 10 bases -- 50 aircraft per base – 1.5 to 1 ratio of operating hours to flying hours
– Radar set has 80% duty cycle relative to aircraft operating hours
– Average of 30 flying hours per aircraft per month– 20 year field use period for each radar system
R M S A
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Blue Flame Radar Support Characteristics
• Maintenance/Logistics Concept
– Organizational --Remove/Replace LRUs on aircraft (10 sites)
– Intermediate -- Remove/Replace SRUs at shop (10 sites)
– Depot -- Repair SRUs (1CONUS site)
• Sparing Concept --Intermediate (LRU & SRU)
--Depot (SRU & Piece Parts only)
• Built-in Test Capability
--Fault isolation to faulty LRU @90%
--Fault isolation to faulty SRU @90%
--Fault detection @ 2%
• Support Equipment
-Organizational -- None
-Intermediate -- Simple PSGE
-Depot --ATE
• All LRUs and SRUs are repairable
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Blue Flame R&M Requirements
• Aircraft MTBM 4.0 hrs. Aircraft MMH/FH (unscheduled) 3.0 hrs. Aircraft MMH/FH (scheduled) 0.5 hrs.• Radar MTBM 20.0 hrs. Radar MMH/FH 0.5 hrs. Radar Failure Rate Allocation Antenna/Receiver LRU 16,667 failures/10x6 hrs Transmitter LRU 20,000 failures/10x6 hrs Processor LRU 10,000 failures/10x6 hrs Displays/Controls LRU 2,500 failures/10x6 hrs Power Supply LRU 883 failures/10x6 hrs• Radar MTTRs & Scheduled Maintenance Organizational level MTTR 0.5 hrs Intermediate level MTTR 2.5 hrs Depot level MTTR 6.0 hrs XMTR Magnetron replacement: Every 1000 flying hrs, 1 person,4.0 hrs.
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Miscellaneous Blue Flame Characteristics
• Spares turnaround time (TAT)– Intermediate level --75 days– Depot level -- 45 days
• Constant failure rate assumed• Re-test OK(RTOK) rate
– Intermediate level -- 20%– Depot level -- 8%
• Learning curve on maintenance -- 90%• One set PGSE per base• Depot ATE availability -- 80% • Ave. administrative delay time -- 0.75 hrs./repair• Ave. logistics delay time -- 6.6 hrs./repair 90% probability of spare in 2.0 hrs.
10% probability of no spare in 48 hrs.
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Blue Flame Aircraft system Elements
WBS Level 0 –Blue Flame Aircraft
WBS Level 1-Major Systems• Airframe• Flight Controls• Navigation• Propulsion• Radar
WBS Level 2-Subsystems (Radar)• Antenna/Receiver• Transmitter• Processor • Display/Controls • Power Supply
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Blue Flame Tradeoff Analyses
• Design Tradeoffs
Baseline -- single transmitter
Alternate -- redundant transmitters (2) – operating redundancy
• Support Tradeoffs
Baseline -- 90% spares assurance
Alternate -- 80% spares assurance
The big question before the house is:Where do we start?
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Review the ‘Blue Flame Case Study’ excel spreadsheet and check/verify the availability numbers corresponding to a single transmitter on the next page and show the results of your analysis.
Blue Flame Aircraft Radar Availability – Case Study
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Radar Availability at stated Spares Level
Design Option 90% Spares 80% Spares
Single Transmitter 65% 55%
Radar Trade Result Summary
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The formula for operational availability, Ao, is:
Therefore we need to obtain only the values of the following terms to evaluate Ao:
MTBMAMTTR(CMT)MTTR(PMT)MTTR(LDT)MTTR(ADT)
Blue Flame Aircraft Radar Availability Solution
MLDT(ADT) MLDT(LDT) MTTR(PMT) MTTR(CMT) MTBMA
MTBMAA0
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90% Spares:
From the given data for a single transmitter, we get:Total Radar Maintenance Action = MTBMA = 20 hoursMTTR(CMT) = 0.5 hours (Organizational Level)MTTR(PMT) = 4 hours (Radar Transmitter Magnetron R/R time)MLDT(ADT) = 0.75 hoursMLDT(LDT) = 0.9*2 + 0.1*48 = 6.6 hours
Since the probability of spare in 2 hours = 90%and the probability of spare in 48 hours = 10%
Plugging these numbers into the formula for Ao gives us:
Therefore Ao = 0.628 = 62.8%
Blue Flame Aircraft Radar Availability Solution
628.085.31
20
75.06.645.002
20Ao
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80% Spares:
All the data is the same as for 90% spares, except for:MLDT(LDT) = 0.8*2 + 0.2*48 = 11.2 hours
Since the probability of spare in 2 hours = 80%and the probability of spare in 48 hours = 20%
Table for comparison of values:90% Spares 80% Spares
Given Value 65% 55%Calculated Value 62.8% 54.9%
Blue Flame Aircraft Radar Availability Solution
%9.54549.045.36
20
75.02.1145.002
20Ao